Vertical cavity surface emitting lasers (VCSEL) include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by a current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
The use of mirror stacks in VCSELs is well established in the art. Typically, mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers. In conventional devices, the number of mirror pairs per stack may range from 20-40 to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
In conventional VCSELs, conventional material systems perform adequately. However, new products are being developed requiring VCSELs which emit light having longer wavelengths. Accordingly, VCSELs emitting light having a long wavelength are of great interest in the optical telecommunication industry. As an example, a long wavelength VCSEL can be obtained by using a VCSEL having an InGaAs/InGaAsP active region. When an InGaAs/InGaAsP active region is used, an InP/InGaAsP material system must be used for the mirror stacks in order to achieve a lattice match to the InP. In this system, however, it is practically impossible to achieve decent DBR based mirrors because of the insignificant difference in the refractive indices in this material system. Many attempts have been made to address this problem including a wafer bonding technique in which a DBR mirror is grown on a separate substrate and bonded to the active region. This technique has had some limited success and also the interface defect density in the wafer fusion procedure causes potential reliability problems.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved long wavelength VCSEL.
Another object of the invention is to provide a reliable long wavelength VCSEL.
Still another object of the immediate invention is to provide for an improvement in the efficiency, optical mode control, and current confinement in a long wavelength VCSEL.
Another object of the present invention is to provide for a decrease in threshold current in a long wavelength VCSEL.
Yet another object of the invention is to provide for a method of fabricating a long wavelength VCSEL that includes a more efficient design.